If you fit a 17X22 into a 22X28 mat (let's say portrait), you get 2.5" left and right borders and 3" top and bottom borders. How would this look with a 1/2" difference in the borders?

Tacky.

However, if you go landscape in this situation--and in all framing with mats--it is preferable to have a slightly larger bottom border provided the other three sides are the same.

SmertZ

IMO, off-setting portrait orientation in this manner works just as well.

Below an example of a "tacky" framing job ramming a 4/3 image into an 18x24 stock frame (the same 4/3 aspect ratio), leaving unequal borders. This was one of my submissions in the Carmel Art Assoc. "The Magic of Pt. Lobos" juried competition 9/28/12., and professional quality presentation was stressed for entries to be accepted.

The judges apparently thought it not too tacky, as I was awarded 2nd overall, and $500 in this highly competitive setting. Eyes of the beholder, I guess...